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From "B. Todd Burruss" <bburr...@real.com>
Subject Re: performance tuning - where does the slowness come from?
Date Thu, 06 May 2010 21:42:56 GMT
i think you will see a slow down because of large values in your 
columns.  make sure you take a look at MemtableThroughputInMB in your 
config.  if you are writing 1MB of data per row, then you'll probably 
want to increase this quite a bit so you are not constantly creating 
sstables.  can't recall, did you see compaction mgr reporting a lot of 
pending compactions?  maybe try to "chunk" your data into multiple 
columns or multiple rows.

i too see slowness that exhibits in the same manner as you guys have 
described.  i'm still trying to track it down as well.

On 05/06/2010 10:56 AM, Ran Tavory wrote:
> Jonathan, I think it's the case of large values in the columns. The 
> problematic CF is a key-value store, so it has only one column per 
> row, however the value of that column can be large. It's a java 
> serialized object (uncompressed) which, may be 100s of bytes, maybe 
> even a few megs. This CF also suffers from zero cache hits since each 
> time a read is for a unique key.
>
> I ran stress.py and I see much better results (reads are < 1ms) so I 
> assume my cluster is healthy, so I need to fix the app. Would 1meg 
> bytes object explain a 30ms (sometimes even more) read latency? The 
> boxes aren't fancy, not sure exactly what hardware we have there but 
> it's "commodity"...
>
> Thanks!
>
> On Thu, May 6, 2010 at 5:22 PM, Jonathan Ellis <jbellis@gmail.com 
> <mailto:jbellis@gmail.com>> wrote:
>
>     columns, not CFs.
>
>     put another way, how wide are the rows in the slow CF?
>
>     On Wed, May 5, 2010 at 11:30 PM, Ran Tavory <rantav@gmail.com
>     <mailto:rantav@gmail.com>> wrote:
>     > I have a few CFs but the one I'm seeing slowness in, which is
>     the one with
>     > plenty of cache misses has only one column per key.
>     > Latency varies b/w 10m and 60ms but I'd say average is 30ms.
>     >
>     > On Thu, May 6, 2010 at 4:25 AM, Jonathan Ellis
>     <jbellis@gmail.com <mailto:jbellis@gmail.com>> wrote:
>     >>
>     >> How many columns are in the rows you are reading from?
>     >>
>     >> 30ms is quite high, so I suspect you have relatively large rows, in
>     >> which case decreasing the column index threshold may help.
>
>     --
>     Jonathan Ellis
>     Project Chair, Apache Cassandra
>     co-founder of Riptano, the source for professional Cassandra support
>     http://riptano.com
>
>


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